Key question in immigration debate: Is U.S. border secure?

The debate over a national immigration overhaul has quickly begun to focus on a key question: Are the U.S. borders secure enough?

Leading Republicans say much more needs to be done to control the illegal flow of migrants from Mexico, and they have vowed not to authorize a path to citizenship for the nation’s 11 million undocumented residents until stricter enforcement provisions are in place.

But the Obama administration contends that it has invested more heavily in enforcement efforts than ever before. Several high-profile studies have found that the federal response has helped reduce the flow of illegal immigrants, bringing net migration between the United States and Mexico to a virtual standstill.

Last year, the government spent $18 billion on immigration control, 24 percent more than all other federal law enforcement agencies combined, according to a studyreleased this month by the nonpartisan Migration Policy Institute. The Obama administration deported nearly 410,000 people in 2012 — a record, and 25 percent more than in 2007.

At the same time, the number of people apprehended at the Mexican border trying to enter the United States dropped to 340,000, a 40-year low, in 2011. Immigration advocates say the decline shows that the heavy investment in border agents and surveillance technology has effectively deterred foreign nationals from attempting to cross.

Obama administration officials also say the government has largely met, and in some cases surpassed, enforcement benchmarks set in 2007, when a comprehensive immigration reform bill failed in the Senate in part because of concerns about border control.

“I certainly feel very comfortable saying that never before has the border been as protected as it is now,” said Nestor Rodriguez, a sociology professor at the University of Texas who has studied immigration from Mexico and Central America. “The large undocumented population is partly a result of that success: People are afraid to go back home because it’s hard to get back in.”

That progress, advocates say, largely renders moot the demand from leading Republicans, including Sen. Marco Rubio(Fla.), that a path to citizenship for illegal immigrants be contingent on stricter border-control measures that remain largely undefined.

“Incredible resources have been spent on the interior and border enforcement, and we need to make sure the road to citizenship is not subject to that,” said Marielena Hincapie, executive director of the National Immigration Law Center. The GOP position “raises the question of whether the border can ever be completely secured and is that road to citizenship even achievable or not?”

The issue is quickly emerging as a key obstacle in the latest round of immigration reform negotiations. Talk show host Rush Limbaugh pressed Rubio on Tuesday to vow he would not support a path to citizenship without tying it to stricter border control.

“If there is not language in this bill that guarantees that nothing else will happen unless these enforcement mechanisms are in place, I won’t support it,” Rubio responded.